So in the first instance, the impact of his words are more mood-setting rather than legalistic. As Camilla Taylor of the LGBT civil rights group Lambda Legal puts it: "The real impact is an expression of leadership that will give a great psychological lift to gay and lesbian couples and their children, and a message to the country that discrimination is not appropriate."

Q: There is Doma, a federal law that relates to gay marriage. What about that?

A: The Defense of Marriage Act, or Doma, was introduced in 1996 and signed into law by President Clinton. In essence, it denies federal respect for gay marriages. What this means is that if a gay couple marries in a state where it is legal, no other state is duty bound to honour that union. Similarly, the federal government itself has no obligation to honour spousal rights of its employees or of its benefits – a provision that causes great hardship in cases where spouses fall sick, have a child or die. It also catches in its clutches multi-national couples, preventing foreign partners of gay or lesbian Americans from applying for immigration status in the US.

The Obama administration has already declared its opposition to Doma, and has actively petitioned courts to strike down the law, so far without success. In that regard, Obama is probably doing all he can at this stage to undermine what is seen within the gay community as a vicious and iniquitous law. He could put his political muscle behind a Bill that is currently pending in both houses of Congress to repeal Doma, but given the Republican control of the House and the slim Democratic grip on the Senate, hopes of a repeal law passing are close to non-existent.

More realistically, the courts could decide that Doma is against the US constitution and tear it up. Two challenges are currently before appeals courts on either coast of the US, and in this sense Obama's backing of same-sex marriage could be significant in setting the tone of the legal debate that most observers expect will soon reach right up to the highest judicial panel, the US supreme court.

A: If Doma were to be repealed, that would change nothing about the central question – should same-sex marriage be allowed in America, as that would remain firmly in the hands of each individual state? Thirty states have passed their own laws forbidding the practice, most recently North Carolina. For anything to change in these states, which cover a huge swathe of the country, a more likely hope for the LGBT community would involve application of the equal protection clause of the US constitution.

This states that anyone is entitled to equal protection under the law, irrespective of his or her race, gender and so on. In a couple of test cases not relating to marriage, the US supreme court has already made it plain that lesbian and gay people are fully entitled to protection under the clause.

The question then becomes: is a refusal to allow same-sex couples to marry an abrogation of their rights under the equal protection clause? Close observers of this area of constitutional law expect the matter also to reach the giddy heights of the US supreme court in the next few years.

An appeals court is already considering the legality of Proposition 8, the Californian referendum that removed the right to same-sex marriage after it had been introduced into state law. Challenges will almost inevitably arise over this week's decision by the majority of North Carolina's voters to ban same-sex marriages, which in turn could reach up to the supreme court.

Q: How would Obama's new public position impact on the US supreme court when the time comes for it to consider this issue?

A: It is here that Obama's public change of heart could be truly significant. So far, the department of justice – the president's legal arm – has declined to get involved in any case involving state-level attitudes towards gay marriage. It has said nothing, for instance, on Proposition 8.

But if, under the new-feel Obama administration, the DoJ begins actively to intervene in such legal challenges to state law, lawyers believe it could prove to be decisive. Shannon Minter, legal director for the National Center for Lesbian Rights, says that if the DoJ were to make its support for gay marriage clear by lodging an amicus brief with the US supreme court when the time comes, the nine justices would undoubtedly listen carefully.

"The president's word does matter, even within the supreme court. The justices have always taken seriously the views of the executive branch – they are not bound by what the president says, but they do show great respect."

Which is not to say that Obama's newly-voiced support for gay marriage would prevail. But it certainly tips the judicial scales a few degrees in that direction.